This invention relates to communications systems in which timing information needed to recover data transmitted over the system must be derived. It is especially suited for timing recovery in systems where phase distortion due to the transmission channel causes asymmetrical distortion of the data waveform.
It is known in a modem data receiver to detect zero crossings of the baseband signal to determine a time reference. A baseband sampling clock can be phase locked to the zero crossings with a one half cycle delay to yield a clock for data sampling at the mid point in the cycle. However, due to transmission distortions and imperfect processing of the received signal at the receiver, the zero crossings may lead the desired sampling point by more or less than one half cycle. Such conditions produce less than optimum results since the sampling time is not at the best point.
The desired time to sample the received data signal is at the maximum signal-to-noise ratio (SNR), sometimes referred to as the maximum eye opening. Even if the sample point could be kept at the mid-cycle point, this will not produce optimum results where phase distortion has caused the data waveform to have a maximum SNR at other than the mid-cycle point.
In U.S. Pat. No. 3,440,548 the choice of the sampling instant for a synchronous digital data receiver is based on the use of the time derivative of the received data signal rather than its threshold crossings. Pattern sensitivity problems are addressed in U.S. Pat. No. 4,797,900.
It is an object of the present invention to provide an improved timing recovery apparatus which uses only selected data information from which to control the data sampling time to maximize performance.